This invention relates, in general, to transmitter and receiver circuits and more particularly to oscillator stages of the transmitter and receiver circuit.
Communication equipment such as cellular telephones have achieved wide spread availability due to increased levels of circuit integration and lower costs. The transmitter and receiver circuit of a communication device, send and receive data or information. It is well known by those skilled in the art that the transmitter circuit "transmits" either directly or via a carrier signal with data transposed thereon for reception by receiver circuits. The transmitter circuit generally outputs a signal of significant power to provide a signal over a wide spread area. Conversely, the receiver circuit is sensitive to a small amplitude signal. If the received signal is a carrier frequency of high frequency, it will be reduced by the receiver circuit to a lower frequency that is easily manipulated within an integrated circuit thereby allowing the information transposed on the signal to be retrieved.
Although transmitter and receiver circuits are different in function, both utilize similar oscillator frequencies. For example, a two stage receiver is tuned to receive a carrier signal of 915 megahertz (MHz). The input signal (915 MHz) is mixed with a first local oscillator signal having a frequency of 1055 MHz. The first stage mixer outputs a difference of the two frequencies (1055 MHz-915 MHz) resulting in a frequency of 140 MHz. A second stage mixer is used to further lower the frequency of the signal. The 140 MHz signal output by the first stage mixer is mixed with a 129.3 MHz signal from a second local oscillator, the difference of the two signals yielding a 10.7 MHz signal. The signal is reduced from 915 MHz to 10.7 MHz so that it is easily manipulated using circuitry formed on a standard integrated circuit process.
An example of a two stage transmitter circuit that would be paired with the previous receiver example outputs a 915 MHz signal for reception by other receiver circuits. Data to be sent is applied to an offset oscillator that outputs 140 MHz. The data or information is transposed on the 140 MHz signal. The 140 MHz signal output by the offset oscillator and the 1055 MHz signal from the first local oscillator (used in the receiver example) is applied to an offset mixer. The offset mixer outputs a difference signal (1055 MHz-140 MHz) of 915 MHz. The 915 MHz signal is then amplified and transmitted. The local oscillator supplying 1055 MHz is always enabled since it is used for both transmitting and receiving signals.
Although close in frequency, the second local oscillator of the receiver circuit outputs a 129.3 MHz signal while the offset oscillator of the transmitter circuit outputs a 140 MHz signal. Since the signal levels received by the receiver circuit are very small, output of the offset oscillator parasitically coupled to the receiver circuit desensitizes the receiver to the desired 140 MHz signal. To alleviate this problem the offset oscillator is removed as an interference source while receiving. A common method for removing the interference generated by the offset oscillator is to operate the offset oscillator at twice the frequency (280 MHz instead of 140 MHz in the example above) and divide the frequency by two using a flip flop. The output of the flip flop and hence the 140 MHz signal can be quickly disabled when the offset oscillator output is not needed. Power dissipation increases as a result of this technique but the interference source is quickly eliminated by moving it to a frequency outside the range of the signal being sensed.
There is a real need for reducing power consumption of a transmitter and receiver circuit for battery powered circuits. Low power translates to extended battery life which is a critical selling point for portable communication gear. Also, a reduction in components used to build the transmitter and receiver circuit would decrease integrated circuit size and reduce cost. It would be of great benefit if a transmitter and receiver circuit is provided having reduced complexity and lower power.